1. Field of the Invention
The present invention relates to a phase-shifting mask and a process for manufacturing the same. More particularly, the invention relates to the structure of a phase-shifting mask which allows image formation of a pattern on a wafer at high resolution in manufacture of semiconductor devices and a manufacturing process of the same.
2. Description of Related Art
A conventional technique of phase-shifting masks is now explained in detail.
FIGS. 3(a) to 3(h) are schematic sectional views illustrating a process for producing a conventional-Levenson type phase-shifting mask (a mask having a section for shifting the phase of light emitted for exposure, referred to as "Levenson mask" hereinafter).
Referring to FIGS. 3(a) to 3(h), a blank mask 2 (a mask only of chromium without a pattern) used for producing a Levenson mask has a two-layered structure of a transparent substrate 1 and a light-tight film 3 formed thereon. Quartz is mainly used as a material for the transparent substrate 1 of the blank mask and chromium is mainly used as a material for the light-tight film 3. The chromium film used for the blank mask 2 is usually about 110 nm thick and is formed by a vacuum deposition method or by a sputtering method.
An electron-beam (EB) resist is mainly used as a material for a protective film required in processing from the blank mask 2 to the Levenson mask. The resist 4 is usually applied onto the blank mask 2 to a thickness of about 500 nm by a spin-on method (FIG. 3(a)). The resist film 4 is patterned by EB writing and development, thereby to form a resist mask. The light-tight film 3 is patterned by dry etching (FIG. 3(b)) using this resist mask.
After the light-tight film 3 is etched, the resist film 4 is removed (FIG. 3(c)).
After the removal of the resist film 4, the resulting blank mask is washed. The resist 4d is applied again to a thickness of about 500 nm. A conductive film 5 is then formed by application to a thickness of about 20 nm (FIG. 3(d)) and EB writing for alignment is conducted (FIG. 3(e)).
The resist film 4d in a recess region of the transparent substrate 1 is selectively removed by washing the conductive film 5 away with water and by development (FIG. 3(f)).
A recess 6 is formed in the transparent substrate 1 by etching to have such depth that the phase of exposure light passing through the recess 6 can be controlled to be inverted by 180 degrees with respect to the phase of exposure light passing through an adjacent transparent section of a pattern. (FIG. 3(g)).
The remaining resist film 4d is removed (FIG. 3(h)).
Through the above-mentioned process, completed is a phase-shifting mask wherein the recess 6 is formed in one of a pair of transparent sections of the pattern adjacent to a light-tight section 7, that is, a Levenson mask.
Next, the principle of the Levenson mask is explained.
FIGS. 4(a) to 4(c) illustrate the principle of the Levenson mask.
As shown in FIG. 4(a), the Levenson mask is provided with a shifter (a section which changes the phase of exposure light by 180 degrees) in one of a pair of regions where exposure light passes. In the case of FIG. 4(a), the shifter is the recess 6 in the transparent substrate 1. Light having passed a region c without a shifter and light having passed a region d with the shifter have the same intensity but their phases are shifted by 180 degrees. Waveforms of light from the light-passing regions have amplitudes in opposite directions at the feet of the waveforms where they overlap each other, and offset each other (FIG. 4(b)). As a result, there is a portion where the intensity of light becomes zero, and accordingly the resolution can be improved. Japanese Unexamined Patent Publication No. HEI 2(1990)-211450 discloses a Levenson type phase-shifting mask of a recess-in-transparent-substrate type having the above-described structure.
Fine patterning by photolithography is limited depending on wavelength of exposure light. However, a stepper (light-exposure device) providing a smaller wavelength of exposure light is far more expensive and requires troublesome preparation. Accordingly, the Levenson mask is used as inexpensive means for overcoming the limit of fine patterning with the wavelength of exposure light unchanged.
However, for performing further finer patterning with the wavelength of exposure light unchanged, the Levenson mask described here is not sufficient, and it is necessary to apply a novel photo mask allowing further enhancement of resolution.